Surface reformation method of high polymer material

ABSTRACT

There is disclosed a surface reformation method of a high polymer material such that by irradiating and excimer-laser beam to only a predetermined area in which electronic parts and the like are temporarily immobilized by a liquid on a substrate which has a high polymer layer on the surface, wettability of the liquid for temporary immobilization only with respect to the predetermined area is improved. After the electronic parts are temporarily immobilized on the substrate by using the method, the electronic parts can be soldered with preferable durability by a fluxess reflow soldering.

This is a divisional application of U.S. Ser. No. 09/028,095, filed Feb.23, 1998 now U.S. Pat. No. 6,017,424 which is a continuation applicationof U.S. Ser. No. 08/619,186, filed Mar. 21, 1996, now U.S. Pat. No.5,801,350

BACKGROUND OF THE INVENTION

The present invention relates to a surface reformation method of a highpolymer material and, more particularly, to a surface reformation methodof a high polymer material which is preferable to be used in order toimprove wettability of a liquid used for temporarily immobilizingelectronic parts on an electronic circuit to board the surface of whichis covered by a high polymer material layer. The invention also relatesto improvement of wettability of a liquid used for temporaryimmobilization of electronic parts when the electronic parts aresoldered on an electronic circuit board according to a fluxless reflowsoldering.

As a conventional technique regarding a surface reformation method forwettability improvement of a high polymer material, for example, methodsusing an O₂ asher, Ar sputter, or these like are known. In the methods,a process is performed by setting a circuit board on which a highpolymer material layer to be processed is coated in a vacuum vessel, andthe surface reformation is executed for the entire surface of the highpolymer material on the substrate.

According to the conventional technique using, for example, the O₂ asheror Ar sputtering mentioned above, the surface of the substrate can bereformed only in an atmosphere of vacuum, and there is a problem suchthat a large-scaled apparatus including vacuum equipment, vacuum vessel,and the like is necessary. According to the conventional technique,since the surface reformation is executed for the entire surface of thehigh polymer material on the substrate, an area which is not necessaryto be processed other than the area on which the electronic parts aretemporarily immobilized on the electronic circuit board or the like isalso reformed. Consequently, the liquid used for the temporaryimmobilization of the electronic parts wets the portion which isunnecessary for the temporary immobilization of the parts, so that thereis a problem such that a large amount of liquid is wasted. As mentionedabove, when the entire surface of the high polymer material on thesubstrate is reformed and the liquid for temporary immobilization wetseven the unnecessary portion, even if the parts are temporarilyimmobilized, the parts are moved from positions of the temporaryimmobilization at the time of a reflow soldering, so that the object ofthe temporary immobilization cannot be achieved. Particularly, when thesize of the substrate is equal to 50 mm×50 mm or larger, since theliquid for temporary immobilization tends to gather in the center andthe temporarily immobilized parts easily move, it is not preferable.Hitherto, a flux for soldering has been commonly used in the temporaryimmobilization of the parts in the reflow soldering and the parts arenot moved from the positions of the temporary immobilization. In case ofperforming the reflow soldering without using the flux, however, whenthe liquid for temporary immobilization wets the unnecessary portion,the object of the temporary immobilization is not achieved as mentionedabove. It is, therefore, conventionally difficult to solder theelectronic parts at predetermined positions on the electronic circuitboard by performing a fluxless reflow soldering.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a surfacereformation method of a high polymer material which solves the problemsof the conventional technique and which can perform a surfacereformation of a substrate in which wettability for a liquid employedfor temporary immobilization is improved with respect to only an areanecessary for temporary immobilization of electronic parts withoutneeding vacuum equipment, vacuum vessel, and the like. The surfacereformation here denotes the reformation of the properties of thesurface for improving the wettability of the liquid for temporaryimmobilization.

The above-mentioned object of the present invention is achieved byirradiating the high polymer material surface with light energy, only atthe area to be treated (that is, an area to be wetted by the liquid fortemporary immobilization.

According to the present invention, by applying the light energy to onlythe area which needs the surface reformation of the present high polymermaterial, the surface of the only portion which is irradiated with thelight energy can be reformed and the wettability for the liquid can beimproved only in the necessary area. Consequently, the liquid fortemporarily immobilizing the electronic parts and the like can make onlythe processed area wet, the processed area can be wetted by a minimumamount of the liquid, and the electronic parts can be certainlytemporarily immobilized at predetermined positions on the surface of thehigh polymer material.

As a base material of a substrate on which the high polymer materiallayer is provided, any substrate used for an integrated circuit such as,for example, a printed board, glass plate, or ceramic plate can be used.

A polyimide system resin is used for the high polymer material layer onwhich the electronic parts are mounted. As the polyimide system resin,polyimide isoindroquinazolindion (PIQ; trade name of Hitachi ChemicalCo., Ltd.) and polymethylmethacrylate (PMMA) can be usually employed. Inaddition, OFPR (trade name of Tokyo ohka Kogyo Co., Ltd.), and B20(trade name of Hitachi Chemical Co., Ltd.), and are also suitable.

A wavelength of the light energy which is thrown onto the surface of thehigh polymer material lies in a range from 100 nm to 600 nm. When thewavelength of the irradiation light is too short, the irradiation lightis transmitted to the inside of the high polymer material. When thewavelength is too long, abrasion of these high polymer material surfaceis insufficient. Both of the cases are not preferable.

An energy density of the irradiation light is set to be larger than 0.03J/cm² and is equal to 0.5 J/cm² or less. When the energy density isequal to 0.03 J/cm² or less, a contact angle of the liquid for temporaryimmobilization is equal to 20 degrees or larger, so that the improvementof the wettablility is insufficient. It is not preferred that the energydensity exceeds 0.5 J/cm², since damage caused to the resin layer isimpermissibly large when the thickness of the polyimide system resinlayer is equal to 0.5 μm or less.

As light energy which satisfies the above conditions, an excimer-laserbeam can be used in the present invention.

Irradiation by the excimer-laser beam is performed by throwing a pulseof 20 ns to 100 ns (pulse duration is 20 ns to 100 ns) at least once.The pulse duration is set to 20 ns to 100 ns, because the pulse durationof the excimer-laser beam is equal to from 20 ns to 100 ns at thepresent technique level, but it is not an absolute condition. The energyof each pulse of the irradiation pulse of the light energy exceeds 0.03J/cm² and is equal to 0.5 J/cm² or less. At least one pulse of the lightirradiation is necessary. Although the number of pulses can be one ormore, the cost increases when the number is set to an unnecessarilylarge number, so that it is usually set to 5 times or fewer.

As a liquid for temporary immobilization, for example, an alcohol systemliquid such as tetraethylene glycol or pentaethylene glycol or an estersystem liquid such as ethyl salicylate can be used.

A proper amount of the liquid for temporary immobilization is adhered toa predetermined area by, for example, dropping the liquid in thepredetermined area. An adhesion amount of the liquid for temporaryimmobilization is set to an amount sufficient to wet the predeterminedarea on the surface of the high polymer material to temporarilyimmobilize the electronic parts and ordinarily set to 4.5 cc to 13.5 ccfor an area of 16 mm×16 mm (that is, 0.017 cc/mm² to 0.053 cc/mm²).

According to the method of the present invention, by temporarilyimmobilizing the electronic parts on the high polymer material layer onthe substrate, the electronic parts can be easily soldered at positionswhere the electronic parts are temporarily immobilized in a step of thefluxless reflow soldering after that.

In order to solder the electronic parts on the substrate, a metalpattern for the soldering can be provided in at least an area to besoldered on the substrate. The metal pattern in this case is constructedby a solderable metal (for example, Cu) and the thickness can be anythickness as long as it is easily soldered. The soldering can beperformed in accordance with, for example, the micro Ball Grid Array(micro BGA) method or CCB (Controlled Collapse Bonding) method. A metallayer of a predetermined shape is provided on the surface of the basematerial of the substrate, and the high polymer material layer havingholes corresponding to predetermined positions of the metal layer iscoated on the substrate on which the metal layer is provided. On theother hand, a solder ball is provided for the electronic part at aposition corresponding to the hole section and the solder ball and themetal layer exposed at the hole section of the high polymer layer arealigned, thereby temporarily immobilizing the electronic parts.

In the method of the present invention using the irradiation light ofthe energy density in the above range, the metal pattern for solderingis not damaged and the surface reformation of the high polymer materialas an insulating material can be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view for explaining a surfacereformation method of a high polymer material in one embodiment of thepresent invention;

FIG. 2 is a section view showing a construction of a substrate in oneembodiment of the present invention;

FIG. 3 is a section view showing a state in which electronic parts aretemporarily immobilized on a substrate in one embodiment of the presentinvention;

FIG. 4 is a schematic perspective view showing a state in which a metalpattern is damaged because of the improper intensity of an irradiationlight;

FIGS. 5a, 5 b, and 5 c are perspective views showing substrates to whichlight energy is applied so that patterns of irradiation areas are asquare, a circle, a shape obtained by gathering and a plurality ofpatterns on the substrate respectively,

FIG. 6a is an explanatory diagram showing a wet state by a liquid in oneembodiment of the present invention;

FIG. 6b is an explanatory diagram showing a wet state by a liquid in theconventional technique;

FIG. 7a is a schematic perspective view showing a step of dropping aliquid for temporary immobilization onto a substrate in order toevaluate the wettability for the liquid used in one embodiment of thepresent invention;

FIG. 7b is a schematic perspective view showing a state in which theliquid is spread after the step of FIG. 7a;

FIGS. 7c, 7 d, and 7 e are schematic perspective views showing states inwhich the liquid is further spread after the step of FIG. 7b, and thewet states of the liquid come to be evaluated;

FIG. 8a is a schematic section view for explaining a contact angle forjudging the wettability for the liquid in another embodiment of thepresent invention; and

FIG. 8b is a graph showing the relation between the contact angle of theliquid for temporary immobilization and the energy density of theirradiation light in the embodiment or FIG. 8a .

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

Embodiment 1 will be described hereinbelow with reference to thedrawings. In the drawings, reference numeral 1 denotes a laserirradiation area; 2 a substrate; 3 a laser beam; 4 a high polymermaterial layer; 5 a base material; 6 an electronic part; 7 a liquid fortemporary immobilization; 8 a metal wiring; 9 an area worked by an O₂asher; 10 a pipette; 11 a wetted area; 12 a solder ball; 13 anelectrode; 14 a metal pattern; and 15 a hole section.

In the present embodiment, as shown by a construction example in FIG. 2,the substrate 2 to be processed in accordance with the surfacereformation method for the high polymer material is constructed in amanner such that the high polymer material layer 4 such as PIQ(polyimide isoindroquinazolindion), PMMA (polymethylmethacrylate), orthe like is coated as an insulating material on the surface of the basematerial 5 such as ceramic or the like. On such a substrate 2, metalwiring necessary for forming an electronic circuit is performed andelectronic parts such as LSI and the like are soldered at predeterminedpositions. The substrate 2 in the present embodiment has a square shapeof 150 mm×150 mm and a thickness of 5 mm. As shown in FIG. 3, the highpolymer material layer 4 is formed on the surface of the substrate basematerial 5 on which the metal wiring 8 is arranged. There are holesections 15 at predetermined positions of the high polymer materiallayer 4. At the positions, the metal wiring 8 is exposed to form themetal pattern 14 to be soldered.

The present embodiment intends to improve the wettability of only anecessary portion on the substrate 2 for the liquid 7 for temporaryimmobilization in case of temporarily immobilizing the electronic parts6 and the like by using the liquid 7 for temporary immobilization whenthe electronic parts 6 such as LSI and the like are soldered.

As shown in FIG. 1, the surface reformation method of the high polymermaterial 4 according to the present embodiment is executed by applyingthe light energy by the laser beam 3 to the laser irradiation area 1 towhich the electronic parts 6 are temporarily immobilized on thesubstrate 2 which is coated by the high polymer material layer 4. As alight source of the laser beam 3, an excimer-laser is used. It isparticularly preferable that the wavelength of the laser beam 3 lieswithin a range from 100 nm to 600 nm and the energy density lies withina range from 0.05 J/cm² to 0.5 J/cm².

When an alcohol system solvent such as tetraethylene glycol,pentaethylene glycol, or the like as a liquid 7 for temporarilyimmobilizing the electronic parts is dropped onto the laser irradiationarea 1 on the surface of the substrate 2 to which the surfacereformation has been performed by irradiating A the laser beam 3, theliquid 7 evenly wetskin the laser irradiation area 1 and does not wetthe unprocessed surface of the substrate 2. The electronic parts 6 suchas an LSI and the like can be temporarily immobilized by the liquid 7 atthe predetermined positions on the substrate 2 to which an embodiment ofthe present invention has been performed before soldering as shown inFIG. 3.

Even in the case where the surface reformation method of the highpolymer material according to the present embodiment is performed to thesurface of the substrate 2 on which the metal pattern 14 exists for thesoldering, the surface of the high polymer material layer 4 coated onthe substrate 2 can be reformed without damaging the metal wiring 8.When the energy density of the laser beam 3 is remarkably larger thanthe values indicated in the invention, for example, equal to 20 J/cm²,the metal wiring 8 is damaged as shown in FIG. 4 and can be broken.

Further, in the method according to the embodiment of the presentinvention, the shape of the laser irradiation area 1 can be optional inaccordance with the sectional shape of the laser beam 3 to be irradiatedas shown in FIGS. 5a, 5 b and 5 c. For example, it can be a square shownin FIG. 5a, a circle shown in FIG. 5b, and a shape obtained by gatheringa plurality of shapes for reforming a wide region shown in FIG. 5c. Themethod according to the present embodiment is effective in theatmosphere, vacuum, or. He assist and can in such circumstance reformthe surface of the high polymer material layer 4. The He assist denotesan atmosphere during spraying He in order to blow off dusts. By sprayingHe, dusts can be blown off further than spraying other gas.

A state of wettability with respect to the liquid when the methodaccording to the foregoing embodiment is performed will be described incomparison with the case of the conventional technique with reference toFIGS. 6a and 6 b.

As shown in FIG. 6a, the liquid 7 as an alcohol system solvent in caseof performing the method according to the embodiment wets only a portionof the area 1 which was irradiated by the laser beam. This is becausethe portion which is not subjected to the process according to thepresent embodiment does not allow the liquid 7 to wet due to the surfacetension even when the liquid is dropped to the not processed portion orthe liquid tries to invade from the area 1 to which the laser beam hasbeen thrown.

On the other hand, in case of the method according to the conventionaltechnique using the O₂ asher, as shown in FIG. 6b, the area 9 to beworked by the O₂ asher cannot be limited to a specific area and theentire surface of the substrate is processed. The liquid 7 consequentlywets the whole surface of the substrate 2.

As mentioned above, in the present embodiment in which only thepredetermined area on the substrate 2 is set to the laser beamirradiation area 1, the liquid 7 for temporary immobilization wets onlythe laser beam irradiation area 1 as shown in FIG. 6a. The electronicparts 6 having the solder ball 12 on the electrode 13 as shown in FIG. 3are mounted on the substrate 2 of which only a predetermined area is wetby the liquid 7 for temporary immobilization, and the electronic parts 6are temporarily immobilized in a manner such that the solder ball 12faces the metal pattern 14 through the hole section 15 of the highpolymer material layer 4. The substrate 2 on which the electronic parts6 are set as mentioned above is reflow soldered, thereby soldering theelectronic parts to the predetermined positions on the substrate. Theflux is not particularly used. The residual liquid for temporaryimmobilization after the completion of the soldering is evaporated byputting the substrate in a vacuum atmosphere.

On the other hand, when the surface of the high polymer material is notespecially processed (when the irradiation of the light energy, the workby the O₂ asher, sputtering, and the like are not performed), thesurface of the high polymer material is not wet by the alcohol systemsolvent, and the electronic parts cannot be temporarily immobilized, sothat the electronic parts cannot be soldered by the fluxless reflowsoldering.

When the surface of the high polymer material is processed by the O₂asher or Ar sputtering as in the conventional technique, the liquid fortemporary immobilization spreads over the entire surface of the highpolymer material and the electronic parts cannot be soldered to thepredetermined positions on the substrate by the fluxless reflowsoldering as mentioned above.

Embodiment 2

A method of evaluating the method according to the present embodiment ofthe present invention will now be described with reference to FIGS. 7ato 7 e. First, the substrate 2 having a square area of 150 mm×150 mm andthickness of 5 mm is prepared as a sample as shown in FIG. 2, and by theexcimer-laser beam having a wavelength of 308 nm, energy density of 0.1J/cm², and pulse duration 30 ns is irradiated once at a 16 mm×16 mmsquare. After that, as shown in FIG. 7a, the liquid 7 of the alcoholsystem solvent, that is, tetraethylene glycol or pentaethylene glycol ofabout 4.5 cc to 13.5 cc, typically about 9 cc is dropped by using thepipette 10. The droplet wets the whole surface in a predetermined areaas shown in FIG. 7b. The droplet spread by wetting is left for 15minutes. When the liquid 7 remains spread on the entire surface of thepredetermined processed area and is not leaked out of the processed areaas shown in FIG. 7c, it is evaluated that the surface is sufficientlyreformed.

On the other hand, a case where it is evaluated that the surface is notsufficiently reformed is shown in FIGS. 7d and 7 e. FIG. 7d shows astate where the liquid 7 wets even out of the predetermined area 1. FIG.7e shows a state where an area wetted by the liquid 7 is reduced and aportion which is not wetted by the liquid 7 appears on the predeterminedarea 1. These surface reformation is insufficient in both of the cases.

In case of irradiating the surface of the high polymer material by thelight energy under the conditions of the present invention, a preferredresult shown in FIG. 7c can be obtained.

Embodiment 3

An experimental result for obtaining the optimum energy density of theexcimer-laser beam according to the present embodiment will now bedescribed with reference to FIG. 8.

The surface reformation can be evaluated by measuring the contact angleθ between the surface of the liquid 7 for temporary immobilization andthe surface of the substrate when the temporary immobilizing liquid isdropped onto the surface of the substrate 2 as shown in FIG. 8a. Whenthe contact angle θ is equal to 20 degrees or less, it can be usuallyevaluated that the surface reformation regarding the wettability issufficient. As it is known, the contact angle is measured byphotographing the side and measuring the contact angle in thephotograph. The surface of the substrate 2 is composed of PIQ. Theirradiation condition of the laser beam, kind of the temporaryimmobilizing liquid, and dropping condition of the temporaryimmobilizing liquid are the same as those in the embodiment 2. Theenergy density of the laser beam is changed in a range shown by the axisof abscissa of FIG. 8b.

FIG. 8b show the experimental result indicating the relation between theenergy density of the excimer-laser beam and the contact angle θmentioned above.

As will be clearly understood from FIG. 8b, when the energy density ofthe excimer-laser exceeds 0.03 J/cm², the contact angle θ becomes 20degrees or less and the effect of the invention can be obtained. Whenthe energy density is equal to 0.05 J/cm² or larger, the contact angle θis sufficiently smaller than 20 degrees and it is further preferable.When the energy density exceeds 0.5 J/cm² ₁ however, it is notpreferable since the damage caused to the high polymer material layerbecomes large as mentioned It, it is preferable that the energy densityis equal to 0.5 J/cm² or less. When the energy density exceeds 0.15J/cm², there is a tendency such that a residue after working increases,so that it is further preferable that the energy density is set to 0.15J/cm² or less.

In the present invention, the energy density of the excimer-laser beamis consequently set to larger than 0.03 J/cm² and is equal to 0.5 J/cm²or less and, preferably, it is equal to 0.05 J/cm² or larger and isequal to 0.15 J/cm² or less.

In the present embodiment, the pulse of the excimer-laser beam for 30 nsis irradiated only once onto the substrate, thereby obtaining the data.When the pulse is irradiated three times or when the irradiation time isset to from 20 ns to 100 ns, it is confirmed that an almost similarresult can be obtained. Further, when PMMA is used for a high polymermaterial, substantially the same result can be also obtained.

When the laser irradiation condition by which the preferred result isobtained in the present embodiment is applied to Embodiment 2, thepreferred result can be also obtained.

According to the surface reformation method of the high polymer materialaccording to the present invention as mentioned above, by applying thelight energy to only the area which needs the surface reformation of thehigh polymer material, the surface reformation of only the portion towhich the light energy is irradiated can be performed and thewettability of the liquid can be improved only in the necessary area.Thus, the liquid for temporarily immobilizing the electronic parts andthe like without needing the vacuum equipment or the like can be wettedonly in the process area and the process area can be wetted with theminimum amount.

According to the invention, since the surface reformation of only thenecessary area can be performed, the electronic parts and the like canbe temporarily immobilized on the substrate by using the minimum amountof the liquid for temporary immobilization.

When the surface reformation is executed by using the surfacereformation method of the high polymer material of the presentinvention, the electronic parts and the like are temporarily immobilizedon the substrate by using the liquid for temporary immobilization afterthat and the fluxless reflow soldering is performed, thereby enablingthe soldering of the electronic parts and the like on the substrate tobe executed with preferable durability.

In the surface reformation method of the high polymer material of theinvention, even when the metal pattern for soldering exists in the lightenergy irradiation area, the metal pattern is not damaged.

What is claimed is:
 1. A circuit board, comprising: a substrate; and ahigh polymer material layer formed on a surface of said substrate,including a first area of said high polymer material layer on which toimplement an electronic part, and a second area different from saidfirst area of said high polymer material layer on which no electronicpart is to be implemented; wherein said first area is an area processedby irradiating a light energy, the wavelength of said light energy beingin a range from 100 to 600 nm, and the energy density of said lightenergy being more than 0.03 J/cm² and less than or equal to 0.5 J/cm²;and the wettability on said first area is greater than the wettabilityon said second area, in accordance with a liquid material contact angleon said first area being smaller than that on said second area.
 2. Acircuit board according to claim 1, wherein the wavelength of said lightenergy is longer than 180 nm.
 3. A circuit board according to claim 1,wherein the light energy is irradiated onto said first area in at leastone pulse of duration 20 to 100 ns.
 4. A circuit board according toclaim 1, wherein the energy density of said light energy is between 0.05J/cm² and 0.15 J/cm² inclusive.
 5. A circuit board according to claim 1further comprising a metal pattern on said substrate below said firstarea of the high polymer material layer.
 6. A circuit board according toclaim 5, wherein said high polymer material layer is a polyimide systemresin layer.
 7. A circuit board according to claim 1, wherein saidliquid material contact angle on said first area is less than 20degrees.
 8. A circuit board according to claim 1, wherein a liquidmaterial having said liquid material contact angle on said first andsecond areas is an alcohol system solvent.